Rotor Faults Diagnosis Way for Induction Motors Based on PQ Transformation

The rotor faults diagnosis method for squirrel cage induction motors based on the PQ transformation is to collect three-phase electric voltage and electric current signal simultaneity, forming the active power P by the product of the currents and the voltage, and the reactive power Q by the product of the currents and Hilbert transformation of the voltages, then diagnose the faults based on the PQ curve of the machine. Due to the P and Q are both constant for healthy motor, they correspond to a dot on the PQ curve. And the PQ curve of machine with rotor broken bar or eccentricity will correspond to an ellipse due to they comprise an additional frequency component. This paper start with the model of the unhealthy squirrel cage induction motors. Then detail analysis indicates that the longest radius and the shortest radius of the ellipse was determined by the effective value of the fault characteristic components in the stator currents of the squirrel cage induction motor, at the same time, and the deflexed angle of the ellipse was determined by the phases of the fault characteristic components in the stator currents of the motor. The change of the amplitude of the longer axis, the short axis and the deflexed angle of the ellipse can be fault diagnostic indexes of the induction motors. Experimental results demonstrate effectiveness of the proposed method.